The traditional use for a staff gauge is to provide a visual way of determining the water level in some body of water (lake, river, flume, weir, etc.). Depending on the type of gauge, the level gradations can be anything desired - feet, meters, inches, yards, etc.
An additional feature of staff gauges is they can also be provided in flow increments - either with level units or in lieu of them. This feature means that spot determinations is the flow rate can be made without the need to use look up tables or flow charts. So long as the flow can be determined by a single level measurement (as is the case with a flume or weir under free-flow conditions) a properly increments staff gauge provide the spot flow rate indication.
For sites where power or flow Instrumentation aren't present or possible, flow incremented staff gauges can be a great boon to an operator.
Naturally, staff gauges only provide an indication of what the flow rate is at the time of inspection, but coupled with an inexpensive video recorder, they can give the operator insight into not only what the flow rate was during the site visit, but also how it trended over time.